Both in physical procedures and chemical reactions, involving an exchange of material between two phases, large contact surfaces and thorough mixing of the phases are just as decisive as long residence times in corresponding contact or reaction zones in order to obtain high turnover and yields. This equally applies to all phase transfers, regardless whether the material exchange is to take place between solid, liquid or gaseous phases.
One possibility for extending contact times and increasing contact surfaces or the number of contacts with discrete particles in the case of solid-liquid and solid-gas contacts consists in leading the two phases to be contacted in countercurrent flow, as it is, for example, described for spray columns, (sometimes multi-staged) fluidized-bed reactors, countercurrent contactors and packed columns by A. W. M. Roes and W. P. M. Van Swaaij, Chem. Eng. J. 17, 81-89 (1979). In DE 10 2007 005 799 A1 (published on 24 Apr. 2008), the countercurrent principle is described as a specific example of pyrolysis reactions and heat transfer processes. Therein, pyrolysis coke is used as a fuel and converted into a product gas which is rich in hydrogen and has a high calorific value, wherein bulk material serving as heat transfer medium is circulated by means of a bulk material conveyer and is conducted in countercurrent flow to the gas stream containing the product gas.
Another possibility for increasing the surface, which is also suggested by Roes and Van Swaaij (supra), provides for internals, which are well known in the field of packed columns or rotating disk columns.
An increase of the residence times in contactors or reactors may, for example, also be achieved by providing flow controllers or restrictors to create zones of differing flow rates of the phases to be contacted with each other. One example of such a fluidized-bed reactor is described in Kersten et al., Chem. Eng. Sci. 58, 725-731 (2003). Therein, a circulating fluidized-bed reactor for biomass gasification is described, which is partitioned into zones of different densities of both the circulating solid and the carrier and combustion gases by means of a regular sequence of conical expansions in the riser, wherein solid particles and gases are conducted in the riser in cocurrent flow. The high velocities in the comparably very tight risers below each conical expansion do not allow the particles to move downwards. This is called spouted beds connected in series. A similar example for improving the flow profile in a fluidized-bed reactor is described by J. Bu and J.-X. Zhu, Canadian J. Chem. Eng. 77, 26-34 (February 1999), where annular internals are provided in the riser of a circulating fluidized-bed reactor, having a similar effect as the conical expansions according to Kersten et al. (supra).
For fluidized-bed reactor systems in which two or more fluidized reactors communicate with each other, the above measures for improving contact or material exchange between two phases, specifically between a solid and a liquid or gaseous phase, is hardly known. For example, Berguerand and Lyngfelt describe in Fuel 87, 2713-2726 (2008) the provision of an alternating arrangement of overflow and underflow weirs in a fluidized-bed reactor system with two fluidized-bed reactors. This arrangement called “particle lock”, however, serves only for separating particles of different densities and not for increasing contact between the particles and the gas phase.
U.S. Pat. No. 3,353,925 discloses several “nozzle-type” contractions in the cross-section of a fast-fluidized reactor in a fluidized-bed system, which in addition comprises two reactors not transporting any particles. These contractions serve to improve contact between gas and particles, which in this case flow concurrently.
In Ind. Eng. Chem. Res 43(18), 5611-5619 (2004), Bi et al. disclose installations called “baffles”, which serve for destroying larger rising bubbles and separating them into smaller ones in a stationary, i.e. bubbling bed, in order to increase homogeneity of the bed.
Finally, the inventors of the present subject matter developed a fluidized-bed reactor system in earlier research work that comprises at least two fast fluidized bed reactors, in at least one of which different reaction zones separated by one or more flow controllers are provided and wherein the particle line for transporting the fluidized-bed particles from other reactors into this one joins it above at least one flow controller (PCT/AT2011/000254). One example for such a system is shown in FIG. 1 herein and will be described in detail later on. The type of flow controller is not particularly limited and any constriction or expansion of the reactor cross-section, deflection of the particle stream or combination thereof can be provided, e.g. a “zigzag” course of the reactor pipe or the provision of various installations, such as e.g. central or lateral baffles, annular constrictions etc., which in addition can be at any angle to the flow direction. The type of flow controller is mainly determined by the intended purpose of the fluidized-bed reactor system and by the respective usable reactor wall material.
The main disadvantage of all known embodiments of flow controllers in fluidized-bed reactors is, however, that after their successful installation in the reactor(s), the flow paths therein are fixed, so that a rearrangement of the reactor system is required for any change, which of course requires the system to be shut down.